In recent years, an increase in size of a wafer is demanded, and a wire saw is mainly used to slice an ingot with this increase in size.
The wire saw is a apparatus that allows a wire (a high-tensile steel wire) to travel at a high speed and presses an ingot (a work) against the wire to be sliced while applying a slurry to the wire, thereby slicing the ingot into many wafers at the same time (see Japanese Unexamined Patent Publication (Kokai) No. 262826-1997).
Here, FIG. 11 shows an outline of an example of a general wire saw.
As shown in FIG. 11, a wire saw 101 mainly includes a wire 102 that slices an ingot, grooved rollers 103 (wire guides) around which the wire 102 is wound, a mechanism 104 that gives the wire 102 a tensile force, a mechanism 105 that feeds the ingot to be sliced, and a mechanism 106 that supplies a slurry at the time of slicing.
The wire 102 is unreeled from one wire reel 107 and reaches the grooved rollers 103 through the tensile-force-giving mechanism 104 formed of a powder clutch (a constant torque motor 109), a dancer roller (a dead weight) (not shown) and so on through a traverser 108. The wire 102 is wound around this grooved rollers 103 for approximately 300 to 400 turns, and then taken up by a wire reel 107′ through the other tensile-force-giving mechanism 104′.
Further, the grooved roller 103 is a roller that has a polyurethane resin press-fitted around a steel cylinder and has grooves formed at a fixed pitch on a surface thereof, and the wire 102 wound therearound can be driven in a reciprocating direction in a predetermined cycle by a driving motor 110.
It is to be noted that such an ingot-feeding mechanism 105 as shown in FIG. 12 feeds the ingot to the wire 102 wound around the grooved rollers 103 at the time of slicing the ingot. This ingot-feeding mechanism 105 includes an ingot-feeding table 111 that is used to feed the ingot, an LM guide 112, an ingot clump 113 for grasping the ingot, a slice pad plate 114, and others, and driving the ingot-feeding table 111 along the LM guide 112 under control of a computer enables feeding the ingot fixed at a end at a previously programmed feed speed.
Moreover, nozzles 115 are provided near the grooved rollers 103 and the wound wire 102, and a slurry can be supplied to the grooved rollers 103 and the wire 102 from a slurry tank 116 at the time of slicing. Additionally, a slurry chiller 117 is connected with the slurry tank 116 so that a temperature of the slurry to be supplied can be adjusted.
Such a wire saw 101 is used to apply an appropriate tensile force to the wire 102 from the wire-tensile-force-giving mechanism 104, and the ingot is sliced while causing the wire 102 to travel in the reciprocating direction by the driving motor 110.
Meanwhile, a wafer sliced out by using the above-explained wire saw 101 may be usually polished and then subjected to epitaxial growth to become a product in case of, e.g., a semiconductor wafer. In the epitaxial growth of a silicon wafer, a silicon single crystal thin film (an epitaxial layer) having a thickness of several μm is grown on a surface of a surface of a polished wafer based on, e.g., chemical vapor deposition (CVD) to improve electrical and physical properties as a wafer, and a device element is fabricated on a surface of this epitaxial layer.
Although there are many combinations of wafers and epitaxial layers, a structure where a P-type epitaxial layer having a regular resistance is grown on a P-type low-resistance wafer is general. A characteristic mark when performing this epitaxial growth lies in that a Bow (Sori) occurs in a wafer after growth as shown in FIG. 13. FIG. 13 shows an example of an epitaxial wafer 221 having an epitaxial layer 223 deposited on a wafer 222.
That is, since the P-type low-resistance wafer 222 contains a large amount of boron (B) having a smaller atomic radius than silicon as a dopant, an average interstitial distance is smaller than that of non-doped silicon. On the other hand, the P-type epitaxial layer 223 with having a regular resistance has a small dopant amount and an average interstitial distance that is relatively larger than that of the wafer. Therefore, when the epitaxial layer 223 is grown on the wafer 222, a Bow change readily occurs in the epitaxial wafer 221 in a direction along which the epitaxial layer 223 becomes convex due to bimetal deformation of both members having the different average interstitial distances.
Incidentally, in an epitaxial wafer having an N-type epitaxial layer with a small dopant amount and a regular resistance grown on an N-type low-resistance wafer containing a large amount of arsenic (As) having a larger atomic radius than that of silicon as a dopant, a Bow change occurs in a direction along which the epitaxial layer becomes concave as opposed to the example depicted in FIG. 13.
Here, FIG. 14 shows an example of a Bow change due to epitaxial growth. In FIG. 14(A), an abscissa represents a Bow value in a wafer (PW) that has been sliced out and then polished but is yet to be subjected to epitaxial growth (or a wafer after slicing), and an ordinate represents a Bow value in an epitaxial wafer (EPW) obtained by performing epitaxial growth to the PW.
Furthermore, FIG. 14(B) is a graph showing a distribution percentage of the PW and the EPW at each Bow value with an abscissa representing a Bow value.
As can be understood from FIG. 14, a correlation of a Bow in the PW sliced out by a wire saw and polished and a Bow in the epitaxial wafer obtained by performing epitaxial growth to this PW (R2=0.94). Moreover, it can be revealed that an increase in Bow due to epitaxial growth is approximately +10 μm (for example, in FIG. 14(A), when a PW Bow is 0 μm, an EPW Bow is 10 μm). It is to be noted that a case where the epitaxial layer side is displaced in a convex direction is defined as a “+” direction here.
On the other hand, considering an epitaxial wafer as a product, minimizing an amount (an absolute value) of a Bow after epitaxial growth is required. It is considered that this minimization can be realized by depositing an epitaxial layer in such a manner that epitaxial growth cancels out a Bow in a wafer serving as a raw material. Therefore, to deposit the epitaxial layer to cancel out an original Bow of the sliced wafer as above, Bow directions (+/−) of the wafer must be aligned in one direction in advance before performing epitaxial growth.
However, when an ingot is sliced out based on a conventional method, Bow directions usually become irregular at respective positions of the ingot in an axial direction. Therefore, when all of the wafers obtained by slicing are measured in a process before polishing and they have Bows in a direction opposite to a desired direction, the wafers must be turned upside down one by one to be put into, e.g., a polishing apparatus in a reversed direction, which is troublesome.